This invention relates to an improved method and apparatus for manufacturing air laid webs of predominantly ligno-cellulosic material, and to the product manufactured thereby. More specifically, this invention relates to a method of manufacturing a low density air laid web wherein the web, after binder application is compacted by a wet surface, thus allowing better controlled penetration of the web by the binder despite the use of significantly less binder. This wet compaction of the web reduces binder content and cost and energy consumption in drying, since less binder is used. The wet compaction also imparts greater tensile strength and delamination resistance to the finished web.
Prior to this invention, the full benefit of air laying of webs was not realized because much of the energy saved in the air laying process was expended to dry the web after it had been impregnated or laden with some type of liquid adhesive binder. Without wet compaction, full binder penetration commonly requires wetting of the web with binder to over 70% total moisture content, at which point most of the liquid binder soaks completely into the air laid web.
Although wetting the web to 70% moisture content and above is desirable in effecting binder penetration, this high level of moisture addition is actually self-defeating, because one of the desired features of the air laid process of web manufacturing is reduced consumption of energy in drying.
In the prior art, unless the air laid web is wetted with binder to over 70% moisture content, a large percentage of the aqueous binder remains superficially on the web surface. Attempts to improve binder penetration by increasing the binder content and reducing overall web moisture content were unsuccessful, because the binder nonetheless tends to stay on the web surface without penetrating to the web interior. Such a web after drying also shows an undesirable crusty texture and low delamination resistance.
Compacting the web after binder application is desirable in effecting binder penetration. A roller compaction on the binder-laden web is usually necessary to distribute the binder more uniformly in the air laid web. However, in the case of a 100% short, cellulosic fiber web, direct roller compaction on a binder-laden web surface is not possible because the short fibers adhere to the compacting roller surface. Even a "teflon" coated compacting roller will have fiber adhesion. A pebbled roller surface reduces the fiber pick up somewhat, but the problem is still not eliminated because binder eventually accumulates on the roller surface and causes fiber adhesion to the roller surface.
The concept of this invention, the use of a wetted fabric-wrapped surface to compact a binder-laden essentially 100% cellulosic fiber web, originates from the observation that the fabric surface in noncontinuous. Consequently, when the fabric surface is wetted with a liquid, the cohesive force between the fabric and the web is less than that between two smooth surfaces in contact. The addition of a surfactant to the liquid wetting the compacting fabric surface lowers its surface tension and further reduces the cohesive force between the compacting fabric and the web, resulting in insignificant fiber pick up during wet compaction.